Described below is an actuator fitted with a motor, a gear unit and an adjustment connection. Such actuators are for example used on valves or butterfly valves in systems for heating, ventilating and cooling buildings. For this and similar purposes the actuators must be reliable, durable and inexpensive, in particular with no expensive components, and capable of being manufactured in large volumes. With the growing importance of wireless control, low-energy operation is also a requirement in the absence of an external power supply. Because of this and in order to protect motor and drive train, such actuators must be switched off or throttled quickly, reliably, with low power and smoothly on reaching a final position or another position to be controlled. The same applies in the case of unexpectedly large loadings in the process of this, for example because of wear and tear to the object to be actuated or because of a foreign body therein. In the case of suitable electric motors, for example synchronous motors or brushless DC motors, damage and a reduction in their service life ensue when they reach their peak-load torque, in particular if they are not switched off quickly.
Because of this the gear unit is fitted with at least two mutually movable gear parts, as well as a spring acting on two of these that counteracts their movement out of an idle state. Flexible springs or torsion springs are standard. Spiral springs are highly suitable, especially in the case of linear motion. The gear unit is designed such that in the event of a blocked adjustment connection the motor moves the gear parts out of the idle state during operation to an operating point where, depending on the application, the motor is switched off, or else run in the opposite direction, or else in the case of a unidirectional motor is throttled in order to retain the position counter to the action of a return spring.
In many such actuators, once the motor is switched off the spring effect sometimes results in the gear parts moving backward and the motor being switched on again. A similar thing can happen when the motor power is throttled. If the actuator starts to splutter, the resulting vibrations can damage the motor, even though its peak-load torque is avoided. An electronic system to reliably prevent this problem requires components that are too expensive for the typical applications.
Other such actuators are marketed under the Siemens brand with type designations SQS or SQX. They are based on the principle of planetary gearing. Part of the gear train is mounted in a separate, pivotable housing. This is held in a defined center position by a torsion spring. When a final position is reached the adjustment connection is blocked and the pivotable housing pivots out of the center position despite torsion of the torsion spring. A cam embodied on the pivotable housing then strikes a switching contact, which interrupts the power supply to the electric motor and thus switches the drive off. The same happens if for another reason a particular toque is exceeded. The electric motor has been selected such that under normal conditions, taking account of manufacturing tolerances, the torsion spring cannot unlock it when standing idle. Thus normally the pivotable housing does not return immediately to the central position, which would switch the electric motor on again and thus make the actuator start spluttering. Only after a new movement command in the opposite direction does the pivotable housing move back to its center position.
Further such known actuators have a similar structure, whereby instead of a torsion spring a flexible spring attempts to hold a pivotable gear part in a center position. The flexible spring is centrally mounted and engages in a recess at the front end of the pivotable housing.